Vehicle engines use various sensors to provide information that is then used to control engine operations for a variety of reasons. One example, U.S. Pat. No. 6,575,148, describes using a specific humidity sensor to modify the engine air-fuel ratio as well as other engine parameters.
The inventors of the present invention have recognized a disadvantage with such an approach. In particular, such a system fails to consider engine misfire effects on the achievable fuel economy performance in controlling engine air-fuel ratio.
Furthermore, when such an engine uses variable cam or valve timing, variations in humidity can further exacerbate engine misfires due to the combined effect of cam timing variation and humidity on engine combustion.
Specifically, the inventors of the present invention have recognized that the achievable valve timing varies as ambient humidity varies. Thus, if valve timing is optimized for low humidity (as much dilution as possible to maximize fuel economy in low humidity conditions), an increase in humidity may cause a change in the mixture dilution thereby increasing potential for engine misfire. Alternatively, when cam timing is set for a worst case of high humidity, thereby reducing engine misfires, this can result in less vehicle economy and increased emissions on low humidity days. As such, operation according to prior approaches results in either increased engine misfires, or lost vehicle fuel efficiency and increased emissions.